The present invention is directed to an optical communication network, and more particularly to a fiber optic network wherein terminal devices are grouped into subnetworks that are connected to a fiber bus in a manner which permits token passing or message collision detection to be localized to the subnetworks, with communication between subnetworks being achieved over the bus via non-interfering channels. The result is a significant reduction in network access time, which becomes independent of the bus length. Both the maximum and average access times are improved.
Continuing improvements in the quality of optical fibers, and in particular reduced attenuation rates, have made optical fiber communication networks an increasingly attractive alternative to networks which employ conductors as the transmission medium. In order to communicate optically, an electrical signal developed within a transmitting terminal device, such as, for example, a telephone, computer, or numerically controlled machine tool, is delivered to an optical transmitter within the terminal device. The optical transmitter uses the electrical signal to modulate light from a source such as an LED or a laser. The modulated light is then transmitted, by optical fibers and splicing devices such as stars and access couplers, to an optical receiver within a receiving terminal device. The optical receiver includes an optical detector, such as a photodiode, which re-converts the modulated optical signal into an electrical signal. Thus the optical transmitters and receivers within the terminal devices, in addition to the optical fibers and other components connecting them, effectively replace conductors which might otherwise have been used. Although optical fibers, like conductors, can convey information in either analog or digital form, the high bandwidth of fibers makes them particularly useful for conveying digital data in serial form.
Fiber optic transmissive stars are passive coupling devices for interconnecting a number of terminal devices in an optical communication network. Depending upon its construction, a star might convey optical signals from a number of terminal devices to a single fiber (an N-to-1 star), from a single fiber to a number of terminal devices (a 1-to-N star), or from and to a number of terminal devices (an M-to-N star). The basic physical structure of a star is illustrated schematically in FIG. 1, wherein four optical fibers have been fused at a tapered region 20 to provide a star 22 having light ports 24, 26, 28, and 30 on one side and light ports 32, 34, 36, and 38 on the other. Light entering star 22 through any of the ports on one side is equally distributed to all of the ports on the other side. For example, light entering port 24 would be conveyed to each of ports 32-38, the light intensity at each of these latter ports being one quarter the intensity of the light originally launched into port 24. Similarly, an optical signal applied to any of ports 32-38 would be conveyed, at reduced intensity, to ports 24-30. Although the example illustrated in FIG. 1 is a 4-to-4 star, in practice stars can have as many as eighty or more pairs of ports. Star 22 could be transformed to a 4-to-1 star by using ports 24-30 to receive optical signals from four terminal devices and using only one port on the other side, for example port 32, to convey light from the star, ports 34, 36, and 38 being terminated in a non-reflecting manner. This same configuration could be used as a 1-to-4 star by conveying optical signals which enter port 32 to the optical receivers in four terminal devices connected to ports 24-30. It will be apparent that 2-to-1 or 1-to-2 access couplers for connecting one optical fiber to another can be fabricated in much the same way.
A number of dispersed stars can be interconnected to form a single network which optically links a number of terminal devices, so that an optical signal from one terminal device propagates to all other terminal devices in the network. In order to permit one terminal device to designate another terminal device as the intended recipient of a message to be transmitted over the network, each terminal device is assigned a unique, identifying address code. The data packet which is transmitted includes an address portion in addition to a message portion in order to permit the addressed terminal device to receive the message portion, which is ignored by the terminal devices which have not been addressed. Sophisticated techniques for generating, detecting, and decoding such data packets have been developed in the electrical communication art and can be readily adapted for use in networks employing optical fibers as the transmission medium.
It frequently happens that more than one terminal device at a time may have a data packet ready for transmission. Since the optical signals would interfere with each other if they were applied to the network simultaneously, a network access control system must be employed to prevent chaos. Several access control systems are available. In the poling system, for example, a central network manager sequentially interrogates the terminal devices in the network by emitting identification codes, and a terminal device with a message to send is permitted to access the network upon receipt of its identification code. Another access control system is called CSMA/CD (carrier sense multiple access with collision detection), frequently referred to as the "contention" or "collision detection" system. In collision detection, each terminal device monitors the network and is permitted to begin transmitting a data packet whenever the network is not already in use. It may happen that two or more terminal devices access the network substantially simultaneously, so that a "collision" occurs. Each terminal device detects the collision and stops transmitting. After a random delay each of the terminal devices is permitted to again seek access to the network.
The "token passing" access control system is similar to the poling system, except that the function of the central network manager is distributed to the terminal devices themselves. Instead of a central network manager which emits a list of terminal device identification codes, the terminal devices themselves emit the codes, or "tokens." In this system a terminal device with access to the network may address any other terminal device and send a message to it, whereupon the terminal device with access "passes the token" to the next terminal device scheduled for access by emitting a code which identifies the next terminal device. The token passing protocol may limit messages to a predetermined length in order to avoid a communication "filibuster" by a terminal device having a long message to send, so that the other terminal devices have an equitable opportunity to access the network at reasonable intervals.
Various token passing schemes are known in the electrical communication art. A typical token passing protocol might provide for a serially transmitted data packet having a clock-synchronizing prefix portion, an address indicator followed by an address portion, a message indicator followed by a message portion, perhaps limited to a predetermined number of bits, and finally a token prefix and the token. The address indicator, address portion, message indicator, and message portion would, of course, be omitted from the data packet if a terminal device merely passes the token along, without sending a message to another terminal device.
Optical signals propagate in fibers at about 200 meters per microsecond, so that the propagation time for a network is about 5 microseconds per kilometer of end-to-end length. It will be apparent that the average time required to gain access to the network, regardless of the network access system employed, increases as the end-to-end propagation time increases. In a collision detection system, for example, the gap between messages must exceed the end-to-end propagation time so that all terminal devices can become aware that a transmission has ended before any terminal device initiates a new transmission. Moreover the duration of the maximum fragment which can be transmitted before a collision is detected and the transmission is aborted is twice the end-to-end propagation time. With a token passing access control system, the token passing period includes the time to propagate the token and process it at the next terminal device on the token list. The time between successive accesses by a terminal device cannot be less, even if no messages are transmitted, than the time to pass the token to all terminal devices on the network. Network size also increases access intervals in a poling network control system.